Stages Of The Estrous Cycle

Proestrus Concentration of progesterone in the blood declines to its lowest level following luteolysis (regression) of the corpora lutea (pl) in the sow or the single corpus luteum (CL) in the mare.[1-3] With regression and transformation of the corpus luteum to nonfunctional corpus albicans on the ovary, systemic blood concentration of progesterone declines, the block to final follicle growth is removed, and the Graafian follicle(s) is permitted to enlarge through stimulation by rising systemic concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary (Fig. 1).

In the sow, a cohort of 10 to 25 antral (fluid-filled) follicles (2 5-mm diameter) continues to develop to ovulatory size[4] while a 25 30-mm dominant follicle continues to enlarge toward the ovulation fossa of the mare's ovary.[2] Gonadotropin (FSH and LH) stimulation triggers production of estrogen from the recruited follicles that initiate a transition in the female's behavior from a nonreceptive to receptive state to mounting (copulation) by the male.

With uterine release of the luteolysin, prostaglandin F2o (PGF2o!) corpora lutea usually undergo regression shortly after day 15 of the sow's estrous cycle[5] and estrogen production by the many developing follicles increases (Fig. 1). Prior to initiation of estrus (standing for mounting by male), the sow's vulva may become swollen and red, she may increase activity, reduce appetite, vocalize (estrual grunts), display increased interest in the male, and attempt to mount other females, but will not stand for mounting herself.

Proestrus in the mare occurs following uterine release of PGF2o to regress the corpus luteum on approximately day 18 of the estrous cycle. With the approach of estrus, the mare may tend to seek the stallion, but not display outward signs of estrous behavior. A nonreceptive mare may display various repelling behaviors to the stallion such as pinning back her ears, kicking, rapidly switching tail, biting, pawing, and squealing.[2,3]

Estrus Concentration of estrogen peaks either shortly before or after the initiation of behavioral estrus expression (Fig. 1). Increased production of estrogen from the multiple 7 8-mm follicles of the sow and 45 50-mm follicle of the mare triggers the preovulatory surge of LH and FSH. Specifically, release of LH induces enzymes to break down the follicle wall and induces ovulation of the oocyte(s).

Expression estrus in pigs averages 24 to 48 h in gilts and 24 to 96 h in sows.[6] The sow's behavioral estrus is characterized by taking a rigged sawhorse-like stance when mounted or having back pressure applied (Fig. 2A). Her ears become erect and she is basically immobile following the tactile stimulation (see video at www.ansi. This standing heat response is more evident in the presence of a boar as stimuli such as boar chanting, tactile nudging, and smell elicit a much stronger expression of estrus in the female. Placing back pressure on the sow will elicit the standing response; however, only approximately 50% of"/>
Fig. 1 Representation of the hormonal changes for the endocrine hormones during the estrous cycle of the sow and mare (day zero is the first day estrus is expressed). (Hormonal profiles adapted from Refs. [2,3,5,7].) (View this art in color at

the females will respond to back pressure alone in the absence of a boar. During estrus, elevated concentrations of estrogen cause the cervix to become very rigid and the long uterine horns to be tightly coiled. Multiple follicles of the sow ovulate over a 1 to 3 h period about 30 35 h after the peak of the LH surge. The LH surge may occur before, during, or after first detection of behavioral estrus, with peak release occurring approximately 12 h from its initial rise. Ovulation usually occurs approximately 70% of the way through estrus. Because the length of estrus is quite variable, determination of the exact time ovulation will occur from initiation of estrus is not a very accurate method to predict time of AI.[6]

Length of the mare's estrus is variable and dependent upon the time of the breeding season. Average length of the mare's estrus is five to seven days, but can range from two to 12 days.[2,3] During estrus, estrogen continues to increase temporally, with the sustained rise (prolonged surge) of LH and FSH during estrus (Fig. 1), and the cervix becomes soft and flat (opposite of the sow). The developing dominant Graafian follicle grows at a rate of about 3 mm/day,[2] reaching a diameter of approximately 40 50 mm prior to ovulation (Fig. 2D). Prolonged length of estrus and LH surge allows the preovulatory follicle to grow toward the ovulation fossa (Fig. 2C). Because the layers (cortex and medulla) of a mare's ovary are inverted compared to other species (thick connective tissue layer of medulla is on the outside), the ovulation fossa is the only site on the ovary that the follicle can release the oocyte. Detection of estrus in the mare is determined through evaluation of behavioral responses following daily exposure to the stallion (teasing). During estrus, the mare displays strong behavioral characteristics of posturing, clitoral winking, and receptivity to teasing by the stallion. Posturing of mare during teasing with a stallion consists of raising her tail and bending her hind legs (squatting) to lower her hindquarters (Fig. 2B). Frequent urination is associated with the squatting response to presence of a stallion. During estrus, the mare displays clitoral winking through the many rhythmic contractions of labia that expose and project the clitoris. A majority of ovulations in the mare occur within two days before the end of estrus expression. Appraisal of estrus and closeness to the time of ovulation on the breeding farm is based on palpation, ultrasonography, and teasing scores (see Table 1) that are utilized to indicate the mare's relative estrous behavior and the level of receptivity to the stallion (see video at The length of estrus expression and intensity of behavior varies greatly among mares. Some mares display very subtle signs of estrus, requiring palpation and/or ultrasonography to optimize time of breeding.

Metestrus Following ovulation of the follicle(s), blood concentrations of estrogen and LH decline rapidly and there is a secondary surge release of FSH caused by the loss of negative feedback of the hormone inhibin (origin Graafian follicle).[1] Rise in FSH following ovulation recruits ovarian follicles into the developing pool for future estrous cycles. After ovulation, the Graafian follicle collapses and blood and lymph seep into the follicle cavity, forming the structure called the corpus hemorrhagicum. Ovulation can be detected with ultrasonography and/or palpation of the ovary. Metestrus is a very short period of the estrous cycle (one to two days), during which the granulosa and thecal cells of the follicle differentiate into progesterone-secreting luteal

Fig. 2 Typical display of behavioral estrus (standing heat) in the sow (A) and mare (B). Photograph of a mare's ovary (C), containing a large Graafian follicle and displaying location of the ovulation fossa. Ultrasonograph of a large (45 mm) mare preovulatory follicle during estrus (D). (View this art in color at

Fig. 2 Typical display of behavioral estrus (standing heat) in the sow (A) and mare (B). Photograph of a mare's ovary (C), containing a large Graafian follicle and displaying location of the ovulation fossa. Ultrasonograph of a large (45 mm) mare preovulatory follicle during estrus (D). (View this art in color at

cells. With growth and expansion of the luteal cells, a functional corpus luteum forms and the first rise in plasma progesterone marks the start of diestrus. Both the sow and mare become rapidly unreceptive to the male following the decline of estrogen and increase in plasma progesterone.

Diestrus With formation of the corpora lutea, concentration of progesterone increases from day three to peak concentrations on day 12 of the sow's estrous cycle. Duration of diestrus is 10 to 12 days. Although tonic release of LH is necessary to support the corpora lutea after day 12 of the estrous cycle, elevated concentrations of progesterone suppress the LH surge and prevent estrus and ovulation. During diestrus, the sow

Table 1 Teasing scores for estrous detection in the mare

Tease score Description

1 Resistant to the stallion (kicking, striking, and squealing)

2 Indifferent (no interest in stallion)

3 Some interest in stallion (stands for teasing without raising tail, winking, or squatting)

4 Responds to teasing by stallion (displays estrus behavior by winking vulva, raising tail, and urinating)

5 Intense expression of estrus behavior (profuse urination, winking vulva, and squatting)

is unreceptive to mounting by the boar. Following 10 to 12 days of progesterone stimulation, the endometrium of the uterus releases the luteolysin PGF2a into the uterine vasculature to initiate luteolysis of the corpora lutea on day 15 of the estrous cycle.[5] With regression of the corpora lutea and decline of progesterone, follicle growth occurs and behavioral patterns of proestrus return.

Diestrus in the mare is initiated with the first increase in progesterone on day nine of the estrous cycle. Progesterone concentrations peak on day 12 and remain elevated until day 18 of the estrous cycle.[2] Duration of diestrus is 14 to 15 days in length, during which the mare will be unreceptive to the stallion. The mare will show disinterest in the stallion (teaser), ears will be back, and she may strike, kick, and squeal in the presence of the male. Uterine release of PGF2a occurs 17 to 19 days after the initiation of estrus expression. Because of the variable length of estrus, the day of ovulation is considered day zero of diestrus on the breeding farm. Thus, luteolysis would occur on days 12 to 15 of diestrus (postovulation). With CL regression, progesterone concentrations decline, follicles grow, and proestrus is initiated.

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